Method for manufacturing non-linear resistors

ABSTRACT

A method for manufacturing non-linear resistors comprising superposing several sintered wafers and applying silver electrodes to opposite surfaces of the sintered wafers superposed respectively is disclosed. The sintered wafer, consisting essentially of iron oxide and copper oxide has negative resistance and also possesses such thermister constant B*K and specific resistance R0 K Omega at 273* K as satisfy the relation B X R0 &lt; OR = 104K Omega .*K. The non-linear resistors manufactured by this method, based on utilization of the property of the contact surfaces between the sintered wafers superposed, have remarkably stable volt-ampere characteristics and are inexpensive in cost.

' United States Patent 11 1 Tachibana et al.

METHOD FOR MANUFACTURING NON-LINEAR RESISTORS lnventors:- Kan-ichiTachibana; Michihiro Nishioka; Mikiya Ono, all of Kitakyushu, JapanMitsubishi Mining & Cement Company, Ltd., Tokyo, Japan Filed: Jan. 26,1973 Appl. No.: 327,177

[73] Assignee:

[30] Foreign Application Priority Data Feb.l6, 1972 Japan... 47-16305US. Cl 29/621,-29/6l0, 338/204,

. 1m. c|.....' H010 1/14, HOlc 17/00 Field of seal-611,. 29/621,:610,612; 338/22, 20, 338/204, 333

References Cited 7 UNITED STATES PATENTS 2,720,573 10/1955- Lundquist'338/22 R- 3,l24,772 .3/1964 Newkirk 29/6l0 X Mar. 5,- 1974 3,343,114 91967 Rice 338/22 R 3,503,029 3/1970 Matsuoka 3,689,863 9/1972 Matsuoka338/20 FOREIGN PATENTS 0R APPLICATIONS 1,446,249 6/1966 France 333 22 RPrimary Examiner-Charles W. Lanham Assistant Examiner-Victor A. DiPalmaAttorney, Agent, or Firm-Brenner & Wray ABSTRACT A method formanufacturing non linear resistors comprising superposing severalsintered wafers and applying silver electrodes to opposite surfaces ofthe sintered wafers superposed respectively is disclosed. The sinteredwafer, consisting essentially of iron oxide and copper oxide hasnegative resistance and also possesses such thermister constant BK a nds pe i tic res i stance R0 K!) at 273 K as satisfy the relation B X R0 510KQK. The non-linear resistors manufactured by this method, based onutilization of the property of the contact surfaces between the sinteredwafers superposed, have remarkably stable volt-ampere I characteristicsand are inexpensive in cost,

2 Claims, 3 Drawing Figures METHOD FOR MANUFACTURING NON-LINEARRESISTORS BACKGROUND OF THE INVENTION This invention relates to a novelmethod for manufacturing non-linear resistors comprising superposingseveral sintered wafers, said sintered wafer having negative resistanceand also possessing the thermistor constant BK and the specificresistance R KQ at 273 K Hastin s/"afaintingremiss (T):

, and applying electrodes to opposite surfaces of said sintered waferssuperposed respectively in a conventional manner. j

In general, the relation between the resistance RKQ at TK and the abovementioned R KQ, BK of an oxide semiconductor is expressed by thefollowing equation R R eXp [B(l/T l/273)] The volt-ampere characteristicof such a varistor is given by the following equation (3):

'I=I,, v/v,, a..........

. I (3) where I is the current flowing through the varistor, V is thevoltage across the varistor, V is the voltage applied to the oppositesurfaces of the varistor at I=I mA, that is, a threshhold voltage, andexponent a, an index representing non-linearity is a numerical valuegreater missus y 3 s Silicon carbide varistors, when made to be smallerin size, deteriorate the characteristics and exhibit a values less thanSilicon varistors and barium titanate varistors are both able to besmall-sized, but are limited in uses because V value is only 0.5 -l .2V.Moreover, in such a varistor which is based on utilization of the mutualcontact between internal particles of a sintered body as siliconcarbidevaristors, it is difficult to completely control the state ofsaid mutual contact between the internal particles even under the samemanufacturing condition, consequently the product is inevitably.variable in the characteristic.

We have found that the defects of the conventional varistors asdescribed hereinabove is able to be overcome by processing conventionalthermister materials (the term sintered oxide material is hereinafterreferred to as thermister) so as to limit the product of the thermisterconstant B K and the specific resistance R,,KQ at 273 K thereof within acertain range.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide a method for manufacturing a varistor which has'a remarkablystable volt-ampere characteristic;

Another object of thisinvention is to provide a method for manufacturinga varistor which is able to be small-sized.

Still another object of this invention is to provide a method formanufacturing a varistor which is inexpensive in cost.

According to this invention, there is provided a method formanufacturing a non-linear resister comprising superposing severalsintered wafers, said sintered wafer having negativeresistance and alsopossessing the thermister constant BK and the specificresistiiiiEFRbKIYflT/FKYBM satisfy the relation PIXRo 10,000 K!) K, andapplying electrodes to opposite surfaces of said sintered waferssuperposed respectively.

This invention isnow explained in detail with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS FIG. 1 is a sectional view ofa varistor manufactured by the method of this invention. I

FIG. 2 shows a volt-ampere characteristic suitable for a varistor.

FIG. 3 shows a volt-ampere characteristic unsuitable for a varistor.

In FIG. 2 and Flg. 3, voltage E is plotted as abscissa and current asordinate.

DETAILED DESCRIPTION OF THE INVENTION This invention, as describedhereinabove, is based on the new information obtained from our studiesthat in case a thermister materialpossesses the thermister constant BKand the specific resistance R KO. at 273 K that satisfy the relation BXR10,000 KO 9 K, a resistor manufactured by superposing several plates ofsuch thermister material shows a stable volt-ampere characteristic asshown in FIG. 2, thus proviing to have an aptitude for a varistor,whereas in case a thermistor material possesses the thermistor constantBK and the specific resistance R KQ at 273 K that satisfy BXR IO,OOOKQ'K, a resistor manufactured by superposingseveral plates of suchthermistor material shows a unstable voltampere characteristic as shownin FIG. 3, thus providing to have no aptitude for a varistOI'.

Therefore, this invention provides a method for manufacturing a varistorcomprising, for example, superposing two sintered wafers 1, 1, eachsintered wafer obtained by processing and sintering a thermistermaterial so as to make the product of BK and R KQ and thereof be notmore than l0,000KQ- K, applying electrodes, for example silverelectrodes 2, 2 to opposite surfaces of the sintered wafers 1, lsuperposed respectively in a conventional manner, attaching lead wires5,

5 to the silver electrodes 2, 2 respectively by using 501- der 4 andcoating over the whole with suitable insulating coating material 6. IThe wafer l is a sintered plate having any one of various shapes such ascircular, square, rectangular, etc.

In this case, no surface barrier exists at the surface of the sinteredwafer 1, which the electrode 2 is applied to, but at the contact surface3, which no electrode is applied to, exists a surface barrier.Therefore, when volt-ampere characteristic as shown in FIG. 2 is. ob-

tained.

This invention also provides a method for manufacturing a non-linearresistor comprising superposing more than three plates of said sinteredwafers and applying electrodes to opposite surfaces of said sinteredwafers superposed in the same manner. The thus manufactured resistorshave remarkably stable volt-ampere characteristics as shown in H6. 2,thus providing to be suitable for a'varistor. 7

As the varistors of this invention have, asrExample shows, remarkablystable volt-ampere characteristics, their yield rate is much higher, ascompared with the conventional methods. Moreover, as the varistors ofthis invention utilize iron oxide, copper oxide, etc. as the main rawmaterials, they are. also advantageous in cost. This invention, asdescribed hereinabove, provides a method for manufacturing varistorshaving remarkably stable characteristics at much higher yield rate andat quite lower cost, therefore it is industrially of great value.

The invention will be understood'more readily withv to be construed tolimit the scope of the invention.

EXAMPLE The sintered body is prepared byaconventional technique. Thestarting material in the composite de fined in Table 1 is respectivelymixed in a potmill so as to produce a homogeneous mixture. The mixtureis dried in a dryer, pressed in a mold at a pressure of about IOOOkg/cminto a disc of 15mm in diameter and 2mm in thickness. The pressed discis sintered in air at about 1,000. C, thus the sintered disc 1 isobtained. A varistor is manufactured by superposing the sintered disc 1upon the other, applying silver electrodes 2, 2 to opposite surfaces ofthe sintered discs ll, 1 superposed, asttaching lead wires 5, 5 to thesilver electrodes 2, 2 by using solder 4 and coating over the whole withepoxy resin 6. As described hereinabove, a surface barrier exists at thecontact surface between the sintered discs 1, 1 which no silverelectrode is applied to. Therefore, when the sintered disc 1 issuperposed upon the other, there generates a high resistance R KQ. Themeasured values of R V 0: of the thus manufactured varistor isrespectively shown in Table 1. InTable l, the V -value is a voltage at1=1mA. Further, theresistance R KQ and the thermisterconstant B K of thesintered disc 1 are measured in a conventional manner, and the values-ofR E and BXR are also shown in Table 1.

TABLE 1 I Composition of Sintered Body Fe 0 (mol%) 66 40 34 CuO (m0l%)34 50 60 66 R, (KO) 0.5 0.1 0.04 0.01 B ("K) 2l00 .1900 H00 1500 R (K9)I30 125 I25 125 'V (V) l9 l9 l9 -l9 As Table l shows, thevalue sofl'i50% of Mastered stable volt-ampere characteristic as shown in FIG. 2,

thus proving to have an aptitude for varistor. The measured values ofthe varistors manufactured by superposing two-four plates of thesintered body consisting of Fe O (50mol percent) and CuO(50mol percent)as anaemia Table 1 are shownin Table 2T Table 2:

Number ofplates 2' 3 '4 12 mm 125 250. 376 v 19.5 33.5 48.0 0z(--) I 5 44 Table 2 shows that varistors of good quality having different V valuescan be manufactured by superpos ing more than'three plates of suchsintered bodies having the product of B and R not more than 10,000KQ'Kas shown in Tablei. I

Next, comparison with Example, the sintered bodies are prepared in thesame manner as Example from the starting materials in the compositionsdefined in Table B, R BXR are also shown in 3. The values of R Table 3.

TABLE 3 Composition of Sintered Body Fe,o,, (mom i 70 o 0 50' MnO,(mul'7l) 0 70 66 0 COO (mol'Ya) 30 30 34 50 R (Kfl) 20 20 l0 6 B (K)4,500 4,500 6,000 3,500 R (KQ) I40 I40 30 50 R XB (Kn' K) 90,000 90,00060,000 2 l 000 As Table 3 shows, the values of BXR of the sinteredbodies are all larger than 10,000KQ' K. The resistor manufactured bysuperposing two plates of each of these sintered bodies showsrespectively a unstable volt-ampere characteristic as shown in FIG. 3,thus proving to be unsuitable for a varistor..

We claim:

1. A method for manufacturing a non-linear resistor comprisingsuperposing several sintered wafers, said sintered wafers havingnegative resistance and also pos- 10,000KQ" K, and applying electrodesto op-

2. A method for manufacturing a non-linear resistor as described inclaim 1, wherein said sintered wafers comprise 66*34 percent of ironoxide (Fe2O3) and 34*66 mol percent of copper oxide (CuO).